Field of the Invention
The present application relates to protective coatings for metal alloys. Specifically, a metal matrix composite consisting essentially of an aluminum alloy and a hard aluminum compound is electro-spark deposited on a substrate, thereby metallurgically bonded to any aluminum alloy substrate.
Description of the Related Art
Coating aluminum alloys without impacting the alloy properties is impossible using typical techniques. State of the art coating processes are not suitable for aluminum alloys because of the following problems related to application of commercially available protective coatings on Al alloys. First, aluminum engineering alloys are often age hardened, and so, any thermal cycle following controlled aging (precipitation heat treatment) can cause over aging, that is, it will weaken the alloy. Conventional coating systems (examples include thermal spray, diffusion processes, cathodic arc PVD, electron beam PVD, cold spray among others) require a post coat thermal cycle to achieve a metallurgical bond (a diffusion bond). Hence, a metallurgical bond cannot be obtained between conventional coating systems and an aluminum alloy without damaging the mechanical properties of the substrate. Second, aluminum forms intermetallic compounds (hard brittle materials) with nearly all other known metals (including all metals in conventional coating systems), and so, any thermal process intended to form a diffusion bond between the coating and the aluminum alloy substrate will result in intermetallic compound formation. The formation of brittle layers in a coating can cause poor mechanical properties, that is, cracking and spalling of the coating. Third, galvanic corrosion can occur whenever different metals in electrical contact are exposed to an aggressive environment. Nearly all components of state of the art protective coating systems will form a galvanic couple with aluminum or aluminum alloys, so rapid corrosion degradation will occur whenever the state of the art materials system (substrate and coating) is exposed to an aggressive environment. Consequently, the most successful coatings for aluminum alloys are mechanically bonded overlay products (primarily thermal spray), but, due to the problems discussed (poor bond strength and galvanic corrosion), these products have limited service life.
Needed then is a metal matrix composite electrode for electro spark deposition (ESD) which can make metallurgical bonded coatings on aluminum alloys without any of the problems encountered with state of the art coating systems.